Close only expansive gate valve

ABSTRACT

A system, including a first flow control element configured to couple to a stem, and a second flow control element coupled to the first flow control element, wherein the first and second flow control elements are configured to expand relative to one another to create a seal in a closed position between a chamber and a flow path in a valve body, and wherein the first and second flow control elements are configured to enable fluid flow between the chamber and the flow path in an open position.

FIELD OF INVENTION

The present invention relates to a gate valve.

BACKGROUND

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present invention,which are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

Valves are used in a variety of applications to manage and transmitflows of materials. Valves generally include an open position thatenables fluid flow through a primary flow path and a closed positionthat reduces or completely shuts off that flow path. However, whentransporting a hot process fluid, for example, over-pressurization of avalve due to thermal expansion of fluids not in the flow path may resultin undesirable wear and/or reduction in the useful life of the valve.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features, aspects, and advantages of the present invention willbecome better understood when the following detailed description is readwith reference to the accompanying figures in which like charactersrepresent like parts throughout the figures, wherein:

FIG. 1 is a partial cross-sectional side view of a gate valve in an openposition according to an embodiment;

FIG. 2 is a partial cross-sectional side view of a gate valve in aclosed position according to an embodiment;

FIG. 3 is a side view of a first flow control element and a second flowcontrol element according to an embodiment;

FIG. 4 is a side view of a first flow control element and a second flowcontrol element according to an embodiment; and

FIG. 5 is a partial cross-sectional side view of a gate valve in an openposition according to an embodiment.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present invention will bedescribed below. These described embodiments are only exemplary of thepresent invention. Additionally, in an effort to provide a concisedescription of these exemplary embodiments, all features of an actualimplementation may not be described in the specification. It should beappreciated that in the development of any such actual implementation,as in any engineering or design project, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which may vary from one implementation toanother. Moreover, it should be appreciated that such a developmenteffort might be complex and time consuming, but would nevertheless be aroutine undertaking of design, fabrication, and manufacture for those ofordinary skill having the benefit of this disclosure.

The disclosed embodiments include a gate valve capable of controllingthe flow of a hot process fluid through a system while blockingover-pressurization of a fluid (e.g. lubricant, stranded process fluid,etc.) within a cavity of the gate valve. As described below, the gatevalve may include first and second flow control elements that transitionbetween open and closed positions to enable or block fluid flow throughthe gate valve. In a closed position, the flow control elements may forma fluid tight seal with one or more valve seats in a valve body, whichblocks the flow of the hot process fluid through the gate valve.However, in an open position, the gate valve prevents the first andsecond flow control elements from forming a fluid tight seal with one ormore valve seats. Accordingly, in an open position, the gate valveenables fluid to ingress and egress from the cavity in the gate valvebody in addition to enabling fluid flow through the gate valve. In otherwords, as the hot process fluid flows through the gate valve, the hotprocess fluid heats and increases the pressure of the fluid in thecavity, but the fluid may escape from the cavity and enter the main flowpath of the hot process fluid, because the first and second flow controlelements selectively do not form a fluid tight seal with the one or morevalve seats in the open position.

FIG. 1 is a cross-sectional view of a gate valve 6 in the open position,allowing fluid flow therethrough. However, in this open position (e.g.,non-sealing position), the gate valve 6 has gaps 8 that enable theingress and egress of fluid (e.g., hot fluid, lubricant) from a cavity10 in a valve body 12 that, as illustrated, is coupled to a valve bonnet14 via one or more bolts 16. An actuator assembly 18, as describedbelow, may be used to move a valve stem 20 along a central axis 22 toactuate the gate valve 6 between open and closed positions (e.g.,sealing and non-sealing positions). In a closed position, the gate valve6 blocks the flow of a hot process fluid with a fluid tight seal. In anopen position, a hot process fluid freely flows through the gate valve6. However, the flow of hot process fluid through the gate valve 6 mayheat a fluid (e.g., a lubricant, stranded hot process fluid, etc.)within the cavity 10. Accordingly, because the gate valve 6 does notform a fluid tight seal in the open position relative to the cavity 10,the gate valve 6 enables the pressurized fluid in the cavity 10 toescape into an inlet passage 26 and/or the outlet passage 28. Thisreduces the likelihood, for example, of over-pressurization of the fluidin the cavity due to thermal expansion of that fluid caused by the hotfluid flow in the gate valve.

As illustrated, the gate valve 6 includes the inlet passage 26 and theoutlet passage 28 with respective flanges 30 and 31 to provideconnections to piping or other components. For example, the gate valve 6may be placed between an upstream pipe 34 transporting a hot processfluid from a source and a downstream pipe 36 transporting the hotprocess fluid to downstream equipment. In such an embodiment, the gatevalve 6 may be used in an on/off manner to allow or block flow from theupstream pipe 34 through the gate valve 6 and into the downstream pipe36. In other embodiments, the gate valve 6 may be used to regulate(e.g., choke) flow from the upstream pipe 34 into the downstream pipe36.

As explained above, the gate valve 6 includes a valve stem 20 (e.g., anelongated rod). The valve stem 20 couples to a first valve controlelement 38. For example, the valve stem 20 may couple to the first valvecontrol element 38 via threading. However, in other embodiments, thefirst valve control element 38 may attach to the valve stem 20 usingother connection joints, such as T-slots, pins, lift nuts, bolts,clamps, welds, and so forth. As illustrated, a second flow controlelement 40 couples to the first flow control element 38 with a rod orwire 32 that weaves between pins 44 on the first and second flow controlelements 38, 40. As illustrated, the rod or wire 32 exerts a collapsingforce on the pins 44 in directions 46 and 48 that clamps the second flowcontrol element 40 to the first flow control element 38. In other words,the rod or wire 32 operates like a tensioned spring, cam, or guide todraw the first and second flow control elements 38, 40 together. In someembodiments, the rod or wire 32 may be non-linear or have a curvedprofile.

The first and second flow control elements 38 and 40 include respectiveports 50 and 52 that selectively allow a hot process fluid to flowthrough the valve body 12, when the first and second flow controlelements 38 and 40 are in an open position. In particular, the ports 50and 52 are openings through the respective second flow control element38 and the first flow control element 40. In FIG. 1, the first andsecond flow control elements 38, 40 are in an open position, so theports 50 and 52 generally align with openings 54 and 56 within an inletseat 58 and an outlet seat 60, respectively, to open the passage 11. Bymoving the first and second flow control elements 38, 40 axially indirections 62 and 64 along the central axis 22, the ports 50 and 52align or misalign with the openings 54 and 56 in the inlet seat 58 andthe outlet seat 60, which enables or blocks the flow of the hot processfluid through the valve body 12 of the gate valve 6. It should beappreciated that the gate valve 6 may be bi-directional, and the terms“inlet” and “outlet” are used for ease of reference and do not describeany specific directional limitation of the gate valve 6. For example,the seats 42, 44 may be either inlet or outlet seats, respectively.

As described above with respect to FIG. 1, the gate valve 6 may includean actuator assembly 18 that opens and closes the gate valve 6 by movingthe first and second flow control elements 38, 40. The actuator assembly18 may include the stem 20, hand wheel 24 (e.g., manual actuator), andbearing assembly 66. In some embodiments, the actuator assembly 18 mayinclude a powered drive system, such as a hydraulic or electric drivesystem, for automatic actuation. As illustrated, the stem 20 extendsthrough an aperture 68 in the bonnet 14. This enables the stem 20 tocouple to the hand wheel 24 (e.g., actuator) and to the first flowcontrol element 38. More specifically, the hand wheel 24 couples to afirst threaded end portion 70 of the stem 20 with a nut 72, while asecond threaded end portion 74 threadingly couples to the first flowcontrol element 38. In operation, an operator opens and closes the gatevalve 6 by rotating the hand wheel 24 (e.g., manual actuator) orengaging a powered drive system to thread the second threaded endportion 74 in and out of the first flow control element 38. As the stem20 threads into the first flow control element 38, the gate valve 6opens as the first flow control element 38 moves in axial direction 64.Similarly, when the stem 20 threads out of the first flow controlelement 38, the gate valve 6 closes as the flow control element 38 movesin axial direction 62. The bearing assembly 66 facilitates rotation ofthe stem 20 by surrounding and aligning the stem 20 in the passageway68. In some embodiments, the bearing assembly 66 rests within acounterbore 76 and surrounds a flange 78 on the stem 20. The gate valve6 retains the bearing assembly 66 and stem 20 within the bonnet 14 witha threaded nut 79 that threads into the counterbore 76.

As explained above, the first and second flow control elements 38, 40form a fluid tight seal in a closed position but block or prevent fluidtight sealing in an open position. Accordingly, in an open position, thegate valve 6 enables a fluid to ingress or egress from the cavity 10 bypassing between the first and second flow control elements 38, 40 andthe inlet seat 58 and the outlet seat 60. The first and second flowcontrol elements 38, 40 include multiple surfaces. These surfaces enablesealing against the inlet seat 58 and the outlet seat 60 in a closedposition, but block or prevent sealing against the inlet seat 58 and theoutlet seat 60 in an open position. For example, the first flow controlelement 38 may include a top surface 80, a bottom surface 82, a seatcontact surface 84, an angled contact surface 86, and a flat contactsurface 88. The second flow control element 40 may include a top surface90, a bottom surface 92, a seat contact surface 94, a first angledcontact surface 96, and a second angled surface 98. As explained above,the actuator assembly 18 moves the first and second flow controlelements 38, 40 between open and closed positions by threading the stem20 into and out of the first flow control element 38.

As the stem 20 threads out of the first flow control element 38, thestem 20 drives the first flow control element 38 in axial direction 62.As the first flow control element 38 moves in direction 62, the angledcontact surface 86 of the first flow control element 38 contacts thefirst angled contact surface 96 of the second flow control element 40,the contact between the two surfaces moves the second flow controlelement 40 in direction 62. As the stem 20 continues to thread out ofthe first flow control element 38, the first and second flow controlelements 38, 40 will continue to move in direction 62 misaligning theapertures 50 and 52 of the first and second flow control elements 38, 40with the apertures 54 and 56 in the inlet and outlet seats 58, 60, asshown in FIG. 2. The misalignment blocks flow through the gate valve 6,but may not form a fluid tight seal. More specifically, as the secondflow control element 40 continues to move in axial direction 62, thebottom surface 92 contacts a bottom surface 100 in a cavity 102 in thevalve body 12. The bottom surface 100 blocks axial movement of thesecond flow control element 50 in direction 62. However, because thefirst flow control element 38 has not contacted the bottom surface 100,the first flow control element 38 continues to move in axial direction38. As the first flow control element 38 moves in direction 62, theangled contact surface 86 slides against the first angled contactsurface 96 of the second flow control element 40. The movement of theangled contact surface 86 along the first angled contact surface 96forces the first and second flow control elements 38, 40 outward inaxially opposite directions 104 and 106 against the respective inletseat 58 and the outlet seat 60, as shown in FIG. 2. The force of thefirst and second flow control elements 38, 40 against the inlet seat andoutlet seat 58, 60 forms fluid tight seals with the inlet and outletsseats 58, 60 and/or gaskets 108 and 110 in the inlet and outlet seats58, 60. In the closed position, fluid is unable to escape from or enterinto the cavity 10.

In order to open the gate valve 6, the stem 20 rotates in an oppositedirection, thereby threading into the first flow control element 38. Asthe stem 20 threads into the first flow control element 38, the firstflow control element 38 moves axially in direction 64 towards the bonnet14. The movement of the first flow control element 38 in direction 64enables the angled contact surface 86 to slide along the first angledcontact surface 96, removing the axial outward force of the first andsecond flow control elements 38, 40 against the inlet and outlet seats58, 60. More specifically, as first and second flow control elements 38,40 move into an open position, the rod or wire 42 compresses the angledsurfaces 72 and 82 inwardly against each other in directions 46 and 48(e.g., the first and second flow control elements 38, 40 retract fromthe previously expanded position). As the first and second flow controlelements 38, 40 continue to move in axial direction 64, the inlet andoutlet seat contact surfaces 94 and 84 slide past the inlet and outletseats 58, 60 aligning apertures 50 and 52 of the first and second flowcontrol elements 38, 40 with the apertures 50, 52 in the inlet andoutlet seats 58, 60, as shown in FIG. 1. In the open position of FIG. 1,the surfaces of the first and second flow control elements 38, 40 do notform fluid tight seals with the inlet and outlets seats 58, 60. Morespecifically, as the stem 20 continues to thread into the first flowcontrol element 38, the top surface 90 of the second flow controlelement 40 contacts a face 112 of the bonnet 14. Contact between the topsurface 90 blocks further movement of the second flow control element 40in direction 64. However, as the first flow control element 38 continuesto move in direction 64, the second angled surface 98 slides over theflat contact surface 88 of the first flow control element 38. In otherwords, the second angled surface 98 on the second flow control element40 does not contact an angled surface on the first flow control element38 that would force the first and second flow control elements 38, 40axially outward in directions 104 and 106. Accordingly, because thefirst and second flow control elements 38, 40 do not move axiallyoutward when the second flow control element 40 contacts the bonnet 14,the first and second flow control elements 38, 40 do not form fluidtight seals with the inlet and outlets seats 58, 60, in an open positionas shown in FIG. 1. Thus, a fluid, such as a fluid heated by the heatedprocess fluid passing through the gate valve 6, may escape the cavity 10through the gaps 8 between the inlet contact surface 94 and the inletseat 58; and between the outlet contact surface 84 and the outlet seat60. By enabling pressurized fluid to escape the cavity 10, the gatevalve 6 blocks over-pressurization of the seals 108 and the bearingassembly 66.

FIG. 2 is a cross-sectional view of a gate valve 6 in a closed positionforming a fluid tight seal 130. As explained above, in the closedposition, the bottom surface 92 of the second flow control element 40contacts the bottom surface 100 of the cavity 102 in the valve body 12.The bottom surface 100 stops axial movement of the second flow controlelement 50 in direction 62, but not the first flow control element 38.The first flow control element 38 can still move in axial direction 38as the angled contact surface 86 slides against the first angled contactsurface 96 of the second flow control element 40. However, the movementof the angled contact surface 86 (e.g., energizing taper, wedge surface,cam surface) along the first angled contact surface 96 (e.g., energizingtaper, wedge surface, cam surface) forces (e.g., wedges, cams,energizes) the first and second flow control elements 38, 40 axiallyoutward in axially opposite directions 104 and 106 against therespective inlet seat 58 and the outlet seat 60. More specifically, themovement of the angled contact surface 86 along the first angled contactsurface 96 enables the first and second flow control elements 38, 40 toovercome the compressive force of the rod or wire 32. As the first andsecond flow control elements 38, 40 move axially outward fluid tightseals 130 form between the first and second flow control elements 38, 40and the inlet and outlets seats 58, 60 and/or gaskets 108 and 110 in theinlet and outlet seats 58, 60. In the closed position, fluid is unableto escape from or enter into the cavity 10.

FIG. 3 is a side view of a first flow control element 38 and a secondflow control element 40 according to an embodiment. The first and secondflow control elements 38, 40 in FIG. 3 may be used in the valve 6 shownin FIGS. 1 and 2. As illustrated, the first flow control element 38 isthe same as the flow control element 38 in FIGS. 1 and 2. However, inFIG. 3, the second flow control element 40 differs from the flow secondflow control element 40 in FIGS. 1 and 2. Specifically, the second flowcontrol element 40 in FIG. 3 includes a flat contact surface 150 insteadof the second angled surface 98 in the second flow control element 40 ofFIGS. 1 and 2. Accordingly, as the gate valve 6 transitions into an openposition, the wire or rod 32 compresses the flat contact surface 150 andthe angled contact surface 96 of the second flow control element 40against the respective flat contact surface 88 and the angled contactsurface 86 of the first flow control element 38, which forms spacebetween the first and/or second flow control elements 38, 40 and theinlet and outlet seats 58, 60.

FIG. 4 is a side view of a first flow control element 170 and a secondflow control element 172 according to an embodiment. The first andsecond flow control elements 170, 172 are interchangeable with the firstand second flow control elements 38, 40 in FIGS. 1 and 2. Asillustrated, the first flow control element 170 includes an aperture174. In an open position, such as shown in FIG. 1, the aperture 174aligns with the apertures 54, 56 in the inlet and outlet seats 58, 60 toenable a hot process fluid to flow through the gate valve 6. However, ina closed position such as shown in FIG. 2, the first flow controlelement 170 blocks the fluid flow through the gate valve 6. The firstand second flow control elements 170, 172 couple together with aconnector bar 176 coupled to pins 178 and 180, which couple torespective flow control elements 172 and 170. Specifically, theconnector bar 176 includes an aperture 182 (e.g., circular aperture) ina first end 184 and another aperture 186 (e.g., elongated aperture orslot) in a second end 187. The apertures 182 and 186 enable theconnector bar 176 to couple to the respective pins 178 and 180, whichcouples the first flow control element 170 to the second flow controlelement 172.

The first flow control element 170 includes multiple surfaces thatenable sealing against the inlet seat 58 and for engaging the secondflow control element 172. For example, the first flow control element170 may include a top angled surface 188, an inlet contact surface 190,a rear surface 192, and a bottom surface 194. The second flow controlelement 172 may include a top surface 196, a front surface 198, anoutlet contact surface 200, and a bottom angled surface 202. Inoperation, the actuator assembly 18 moves the first and second flowcontrol elements 170, 172 between open and closed positions.Specifically, as the stem 20 threads out of the second flow controlelement 172, the second flow control element 172 moves in direction 62.As the stem 20 continues to thread out of the second flow controlelement 172, the first and second flow control elements 170 and 172 willcontinue to move in direction 62, which misaligns the aperture 174 withthe inlet seat 58 and outlet seat 60. The misalignment blocks flowthrough the gate valve 6, but may not form a fluid tight seal. As thesecond flow control element 172 continues to move in axial direction 62,the bottom surface 194 of the first flow control element 170 willcontact a bottom surface 100 of cavity 102 (seen in FIGS. 1 and 2),which blocks further axial movement. When the first flow control element170 contacts the bottom surface 100, the first flow control element 170stops moving in direction 62, but not the second flow control element172. As the second flow control element 172 continues to move in axialdirection 62, the pin 180 slides within the aperture 186 of theconnector bar 176 enabling the bottom angled surface 202 to contact andslide along the top angled surface 188 of the first flow control element170. As the angled contact surfaces 188 and 202 slide against and alongeach other, the surfaces 188 and 202 force the first and second flowcontrol elements 170, 172 axially outward in direction 104 and 106. Thisforces the first flow control element 170 into contact with the inletseat 58 forming a fluid tight seal that blocks fluid flow through thegate valve 6. However, because the second flow control element 172 doesnot overlap the rear surface 192 of the first flow control element 170,the second flow control element 172 may contact, but does not form afluid tight seal with the outlet seat 60.

The gate valve 6 opens as the stem 20 threads into the second flowcontrol element 172. The movement of the second flow control element 172in direction 64 enables the bottom angled surface 202 to slide along thetop angled surface 188, removing the axial outward force of the firstand second flow control elements 170, 172 against the inlet and outletseats 58, 60. As the second flow control element 170 continues to movein axial direction 64, the bottom angled surface 202 of the second flowcontrol element 172 may separate from the top angled surface 188 of thefirst flow control element 170 to open an intermediate gap 203. Asexplained above, the connector bar 176 couples the first and second flowcontrol elements 170, 172. Accordingly, as the second flow controlelement 172 moves in axial direction 64, the connector bar 176 moves thefirst flow control element 172 into an open position (i.e., aligning theaperture 174 with the aperture 54 in the inlet seat 58). In the openposition, the surfaces of the first and second flow control elements170, 172 do not form fluid tight seals with the inlet and outlets seats58, 60 enabling a fluid in the cavity 10 to escape by passing betweenthe front surface 190 and the inlet seat 58; and between the rearsurface 192 and the outlet seat 60. Accordingly, the gate valve 6 blocksoverpressurization of the seals 108 and the bearing assembly 66.

FIG. 5 is a cross-sectional side view of the gate valve 6 with the firstand second flow control elements 38, 40 in an open position. In FIG. 5,the first and second flow control elements 38, 40 are reversed with thefirst flow control element next to the inlet seat 58 and the second flowcontrol element 40 next to the outlet seat 60. More specifically, theseat contact surface 84 of the first flow control element 38 contactsand seals with the inlet seat 58 in a closed position, while the sealingsurface 94 of the second flow control element 40 seals with the outletseat 60. As explained above in FIGS. 1 and 2, in the closed position thebottom surface 92 of the second flow control element 40 contacts thebottom surface 100 of the cavity 102. The bottom surface 100 stops axialmovement of the second flow control element 40 in direction 62, but notthe first flow control element 38. The first flow control element 38 canstill move in axial direction 38 as the angled contact surface 86 (e.g.,energizing taper, wedge surface, cam surface) slides against the firstangled contact surface 96 (e.g., energizing taper, wedge surface, camsurface) of the second flow control element 40. The movement of theangled contact surface 86 along the first angled contact surface 96forces (e.g., wedges, cams, energizes) the first and second flow controlelements 38, 40 axially outward against the respective inlet seat 58 andthe outlet seat 60. The force of the first and second flow controlelements 38, 40 against the inlet seat and outlet seat 58, 60 formsfluid tight seals with the inlet and outlets seats 58, 60 and/or gaskets108 and 110 in the inlet and outlet seats 58, 60. In the closedposition, fluid is unable to escape from or enter into the cavity 10.

However, the first flow control elements 38 in FIG. 5 differs from thefirst flow control element 38 in FIGS. 1 and 2. Specifically, the firstflow control element 40 in FIG. 5 includes an angled contact surface 210and a flange 212. In operation, as the stem 20 threads into the firstflow control element 38, the first flow control element 38 moves axiallyin direction 64 towards the bonnet 14. The movement of the first flowcontrol element 38 in direction 64 enables the angled contact surface 86to slide over the first angled contact surface 96, which reduces theaxial outward force of the first and second flow control elements 38, 40against the inlet and outlet seats 58, 60. Furthermore, as the firstflow control element 38 moves in direction 64, the angled contactsurface 210 contacts the second angled surface 98, which moves thesecond flow control element 40 in direction 40. Accordingly, the gatevalve 6 may not include a rod or wire 32 or pins 44 that couple thefirst and second flow control elements together. The first flow controlelement 38 may continue to move in direction 64 until the flange 212contacts the face 112 of the bonnet 14. Without the flange 212, the topsurface 90 of the second flow control element 40 would contact thesurface 212 of the bonnet 14 enabling the angled contact surface 210 ofthe first flow control element 38 to contact the second angled surface98 of the second flow control element 40 forcing the first and secondflow control elements 38, 40 radially outward in directions 104 and 106.As the first and second flow control elements 38, 40 slide past eachother in axial directions 104 and 106, the flow control elements 38 and40 would form a fluid tight seal with the inlet and outlet seats 58 and60 in an open position without the flange 212. As explained above, afluid tight seal between the first and second flow control elements 38,40 with the inlet and outlet seats 58, 60 would block fluid fromescaping the cavity 10. In other words, a fluid tight seal in the openposition would block pressure relief of a fluid in the cavity 10 as thehot process fluid flows through the gate valve 6 heating and increasingthe pressure of the fluid in the cavity 10. Accordingly, the flange 212blocks contact between the second flow control element 40 and the bonnet14 and, therefore, prevents the first and second flow control elements38, 40 from expanding radially outward against the inlet and outletseats 58, 60 and closing the gaps 8.

In some embodiments, the gate valve 6 may not include a flange 212 thatblocks sealing of the cavity 10 when the first and second flow controlelements are in an open position. For example, in some embodiments, awidth 214 of a second bonnet counterbore 216 may be greater than a width218 of the first and second flow control elements 38, 40 when coupledtogether. The width 218 therefore blocks contact between the second flowcontrol element 40 and the bonnet 14, which prevents the first andsecond flow control elements 38, 40 from expanding into sealing contactwith the inlet and outlet seats 58, 60 in an open position. In otherembodiments, the second flow control element 40 may define a length 220that blocks the top surface 90 from contacting the bonnet 14 when thestem 20 is completely threaded into a stem aperture 222 of the firstflow control element 40. In another embodiment, the stem aperture 222may define a depth 224 that blocks contact between the second flowcontrol element 40 and the bonnet 14 when the stem 20 is completelythreaded into the stem aperture 222. In still another embodiment, thefirst and/or second flow control element 38, 40 may include a passage226 that allows fluid communication between the cavity 10 and thepassage 11 through the gate valve 6, when the first and second flowcontrol elements 38, 40 are in an open position. In still anotherembodiment, the inlet seat contact surface 94 or the outlet seat contactsurface 84 may include a groove that enables fluid communication betweenthe cavity 10 and the passage 11 when the first and second flow controlelements 38, 40 are in an open position.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. A system, comprising: a valve, comprising: a valve body having achamber and a flow path along a first axis; a stem extending along asecond axis; and a flow control assembly coupled to the stem, whereinthe stem is configured to selectively move the flow control assemblythrough the chamber between a closed position and an open positionrelative to the flow path, and the flow control assembly comprises: afirst flow control element; and a second flow control element, whereinthe first and second flow control elements are configured to expandrelative to one another to create a seal between the chamber and theflow path in the closed position of the flow control assembly, whereinthe first and second flow control elements are configured to contractrelative to one another to open a gap between the chamber and the flowpath in the open position of the flow control assembly.
 2. The system ofclaim 1, wherein the flow control assembly comprises a cam interfaceconfigured to gradually expand the first and second flow controlelements relative to one another as the flow control assembly moves fromthe open position to the closed position.
 3. The system of claim 2,wherein the cam interface comprises a first tapered surface of the firstflow control element and a second tapered surface of the second flowcontrol element.
 4. The system of claim 2, wherein the flow controlassembly comprises a biasing structure configured to help contract thefirst and second flow control elements relative to one another as theflow control assembly moves from the closed position to the openposition.
 5. The system of claim 4, wherein the biasing structurecomprises a spring.
 6. The system of claim 4, wherein the biasingstructure comprises a curved guide.
 7. The system of claim 6, whereinthe curved guide is disposed along a first guide pin on the first flowcontrol element and a second guide pin on the second flow controlelement.
 8. The system of claim 6, wherein the curved guide is disposedalong a third guide pin on the second flow control element.
 9. Thesystem of claim 6, wherein the curved guide couples together the firstand second flow control elements.
 10. The system of claim 3, wherein thefirst and second flow control elements at least partially overlap oneanother in an axial direction along the second axis.
 11. The system ofclaim 3, wherein the first and second flow control elements do notoverlap one another in an axial direction along the second axis.
 12. Thesystem of claim 11, comprising an axial joint extending along the secondaxis and coupling to the first and second flow control elements.
 13. Asystem, comprising: a first flow control element configured to couple toa stem; and a second flow control element coupled to the first flowcontrol element, wherein the first and second flow control elements areconfigured to expand relative to one another to create a seal in aclosed position between a chamber and a flow path in a valve body, andwherein the first and second flow control elements are configured toenable fluid flow between the chamber and the flow path in an openposition.
 14. The system of claim 13, wherein the first flow controlelement comprises a first portion with a first angled surface and thesecond flow control element comprises a second portion with a secondangled surface, and wherein the first angled surface is configured toengage the second angled surface to axially expand the first flowcontrol element and the second flow control element to form the seal.15. The system of claim 13, wherein the first flow control elementscomprises a third portion with a third surface and the second flowcontrol element comprises fourth portion with a fourth surface, andwherein the third surface and the fourth surface are configured to slidepast each other without forcing the first flow control element and thesecond flow control element to expand axially.
 16. The system of claim13, wherein the first flow control element comprises a flange configuredto engage a bonnet to block expansion of the first and second flowcontrol elements in the open position.
 17. The system of claim 13,wherein the second flow control element comprises a truncated first endconfigured to prevent contact between a bonnet and the second flowcontrol element when the first flow control element is completelyretracted within a counterbore of the bonnet.
 18. A system, comprising:a valve body having a flow path and a chamber; a bonnet coupled to thevalve body comprising a passage and a flow control element counterbore;a stem extending through the passage; a first flow control elementcoupled to the stem; and a second flow control element coupled to thefirst flow control element, wherein the first and second flow controlelements are configured to expand relative to one another to create aseal in a closed position between the chamber and the flow path in thevalve body, and wherein the first and second flow control elements areconfigured to contract relative to one another to open a gap between thechamber and the flow path in an open position.
 19. The system of claim18, wherein a diameter of the counterbore is greater than the diameterof the first flow control element and the second flow control elementwhen coupled together in the open position.
 20. The system of claim 18,wherein a stem aperture in the first flow control element defines adepth configured to prevent contact between the second flow controlelement and the bonnet when the stem is completely inserted into thestem aperture of the first flow control element.